A lucky fossil find and a novel imaging technique have brought researchers closer to understanding the mysterious origins of the sea spider, a spindly arachnid that dwells on the ocean floor.
Sea spiders look much like the common "daddy long leg" spiders, with thin, long legs and a tiny torso. Although they seem to be related to land spiders, they also possess some unusual anatomy--including special compartments on their heads for carrying eggs--that suggest they may deserve their own branch of the arachnid family tree. Scientists have struggled to solve the mystery, in part because the delicate sea spiders are rarely preserved as fossils.
That's why University of Oxford paleobiologist Derek Siveter was elated when he and colleagues found the oldest and most complete sea spider fossil to date in Herefordshire, United Kingdom. The fossil, which dates back some 425 million years, was too delicate to excavate by traditional means, so the researchers took digital images of the fossil at 20-micrometer intervals as they ground through the surrounding rock. The team reconstructed the slices in a computer, creating highly detailed, three-dimensional pictures of the specimen that can be rotated or dissected at will, the researchers report in the 21 October issue of Nature. The images have led Siveter to conclude that sea spiders are close relatives of land spiders after all. The presence of pincerlike claws called chelae means that they should be grouped with spiders, scorpions, and horseshoe crabs, all of which possess the distinctive feature. The fossil also "has all the hallmarks of current-day sea spiders," Siveter says, suggesting that sea spiders emerged as a distinct group about 450 million years ago. Siveter's team has produced a "a jaw-droppingly beautiful representation" of the fossil, says Rich Palmer, an invertebrate zoologist at the University of Alberta, Edmonton. "The details are astounding," he says. The study "fills a gap in the fossil record with an extremely well-preserved specimen" and may provide valuable clues about a species that has been "virtually ignored by zoologists," adds Jason Dunlop, curator for arachnids at the Museum für Naturkunde der Humboldt-Universität in Berlin, Germany.